1. Field of the Invention
The invention relates to a driving force transmission apparatus that transmits driving force from an input shaft to an output shaft, and a controller that controls the driving force transmission apparatus.
2. Description of Related Art
There is a conventional driving force transmission apparatus that is mounted in, for example, a four-wheel-drive vehicle, and in which a first rotary member and a second rotary member are connected to each other by a clutch such that driving force is transmittable therebetween (refer to, for example, Japanese Patent Application Publication No. 2007-78134 (JP 2007-78134 A)).
The driving force transmission apparatus (driving force distribution apparatus) described in JP 2007-78134A includes a first rotary member (rear wheel output shaft), a second rotary member (clutch drum), a clutch and a cam mechanism. The second rotary member is disposed coaxially with the first rotary member so as to be rotatable relative to the first rotary member. The clutch is disposed between the first rotary member and the second rotary member. The cam mechanism converts the torque generated by an electric motor (servomotor) into pressing force (thrust force) headed toward the clutch. The first rotary member is directly connected to an input shaft into which driving force generated by an engine is input via a transmission. The second rotary member is connected to a front wheel output shaft via a chain belt.
The cam mechanism includes a first ball cam, a second ball cam and a drive gear. The first ball cam has an external gear. The second ball cam has an internal gear. The drive gear meshes with both the external gear and the internal gear. The cam mechanism is configured such that the drive gear is driven by the electric motor via a speed reducer. As the drive gear is rotated upon reception of the torque from the electric motor, relative rotation between the first ball cam and the second ball cam generates the thrust force that presses the clutch in the axial direction.
When the driving force from the engine is input in the first rotary member via the input shaft, the first rotary member rotates about its axis. When the driving force from the engine is distributed to the front wheel side via the clutch, the electric motor is energized to operate the cam mechanism. Upon reception of the torque from the electric motor, the cam mechanism generates the thrust force that presses the clutch. As the thrust force is applied to the clutch, the first rotary member and the second rotary member are connected to each other such that the driving force is transmittable therebetween. Thus, the driving force from the engine is transmitted from the input shaft to the front wheel output shaft via the driving force transmission apparatus.
The driving force that is transmitted from the first rotary member to the second rotary member is controlled by increasing or decreasing current that is supplied to the electric motor, thereby adjusting the thrust force that is generated by the cam mechanism. However, due to variations in the friction resistance in the speed reducer, variations in the output characteristics of the electric motor, or variations in the characteristics of a controller that supplies current to the electric motor, there is a possibility that the driving force corresponding to the current supplied to the electric motor will not be transmitted via the clutch.
In the light of the above-described circumstances, the inventors of the present invention came up with an idea that the thrust force that is generated by the cam mechanism is accurately controlled by detecting a reaction force, which is generated by reaction of the thrust force generated by the cam mechanism, using a sensor. However, for example, if an impact is applied to the sensor due to sudden rotation of the electric motor, the sensor is damaged and thus the reaction force is not accurately detected in some cases.